Control mode switching system for a servomechanism

ABSTRACT

A track and store amplifier maintains a constant direct current error voltage to the servomechanism while transferring from one mode of control to another. A comparator supplies a logical 1 or 0 to a digital computer to generate a positive or negative command slew in predetermined increments to force the new direct current voltage to a value such that it equals the direct current error voltage that has been stored prior to the transfer. In this way, a smooth transition from one mode of control to another is realized.

United States Patent [191 Gross et al.

[ June 26, 1973 CONTROL MODE SWITCHING SYSTEM FOR A SERVOMECHANISM Inventors: Alan E. Gross, Minnetonka; Rodney L. Larson, St. Louis Park; Ralph E. Skoe, Minnetonka, all of Minn.

[73] Assignee: MTS Systems Corporation,

Minneapolis, Minn.

[22] Filed: July 14, 1971 [21] Appl. No.: 162,561

[52] US. Cl. 318/591, 318/600, 91/42 [51] Int. Cl. G051) 7/00 [58] Field of Search 318/591, 600; 91/42 [56] References Cited UNITED STATES PATENTS 3,576,535 4/1971 Turner 318/591 X 3,549,976 12/1970 Bretagne.... 318/591 3,422,457 1/1969 Koppel 318/591 2,913,146 11/1959 Dickerson 318/591 X 3,363,157 l/1968 Rouvalis 318/591 X OTHER PUBLICATIONS IBM Technical Disclosure Bulletin, V01. 12, No. 1, June 1969, CC. Liu, Supervisory Analog Controller station.

Primary Examiner-T. E. Lynch Attorney-Ralph L. Dugger, Nickolas E. Westman et a1.

[57] ABSTRACT A track and store amplifier maintains a constant direct current error voltage to the servomechanism while transferring from one mode of control to another. A comparator supplies a logical 1 or 0 to a digital computer to generate a positive or negative command slew in predetermined increments to force the new direct current voltage to a value such that it equals the direct current error voltage that has been stored prior to the transfer. In this way, a smooth transition from one mode of control to another is realized.

13 Claims, 1 Drawing Figure CONTROL MODE SWITCHING SYSTEM FOR A SERVOMECHANISM BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to servomechanisms, and pertains more particularly to a control system for effecting a smooth transition from one mode of control to another.

2. Description of the Prior Art Present-day techniques involve three basic steps in a computer-controlled mode switching operation. First, the feedback voltage level of the new control parameter is read into the computer through an analog-todigital converter. Next, the actual feedback voltages to the servo controller are switched. In a matter of microseconds later, the required command voltage level to match the new feedback is put out to the servo controller via a digital-to-analog converter. Any errors due to improper loop gains, zero offsets, and A/D or D/A conversions cause an abrupt acceleration to occur with respect to the servomotor or actuator. When conducting vibra-tion tests, such an acceleration can shock the specimen so severely that the integrity of the test is completely destroyed or even the specimen itself is destroyed.

SUMMARY OF THE INVENTION Accordingly, one object of the present invention is to provide a smooth or bumpless transition between the control modes, thereby reducing or eliminating the system errors during a mode switch.

Another object is to effect the transfer in a minimum amount of time.

A further object of the invention is to prevent transients from influencing the degree of control during a switching from one mode to another.

Another object of the invention is to keep the transfer time to an interval much less than the propagation time of most servovalves. In this regard, although the invention will find utility in the control of electromagnetic servo-motors, the invention nonetheless will find especial utility in connection with the control of hydraulic servomotors. Consequently, when following the teachings of the present invention, a transfer is made quickly enough so as not to delay the continuation of any tests when a hydraulic servomechanism is involved.

A further object of the invention is to keep switching transients out of the control loop by using a track and store amplifier. In other words, by reason of the open loop control exercised during the actual mode switching, transients are prevented from reaching the hydraulic system where such a system is employed, or if the system is electrical, the transients are prevented from reaching the electrical system.

As far as a hydraulic servo system is concerned, another object of the invention is to prevent the servovalve from changing its orifice opening during a switch from one mode to another. In this way, the hydraulic system is compelled to maintain its instantaneous velocity during a mode switch.

Yet another object of the invention is to provide a control mode switching system for a servomechanism in which the adaptive command revisions cause a near zero amplitude change in the new control parameter.

Another object of the invention is to provide means for effecting a mode switching for a servomechanism in which a conventional control can readily be modified so as to incorporate thereinto the teachings of the present invention.

Still another object of the invention is to provide a control system for a servomechanism that effects the control mode switching with simple electrical circuitry which is not only inexpensive but is highly reliable.

Briefly, a track and store amplifier is employed so as to, in effect, remember the DC error voltage just prior to initiating a mode transfer action. The stored or held error voltage is utilized for the purpose of establishing an open loop control of the servomechanism during the actual mode switching. Consequently, it is important to effect the changeover just as rapidly as possible. Accordingly, a digital computer is included in the overall combination which outputs the new command, the new command being converted to an analog signal by a digital-to-analog converter. A servo controller compares the analog command signal with an existing feedback signal and provides a new DC error voltage. A comparator continually compares this new DC error voltage with the stored DC error voltage and causes the computer to slew its command so that the new DC error voltage is changed or adjusted so that it equals the stored DC error voltage. This is accomplished in specific steps or increments determined by the clock frequency of the computer. To provide the new command, an analog-to-digital converter first reads a digital signal corresponding to the transducer voltage into the computer. The computer subtracts the transducerderived digital signal from the present digital command signal, thus producing the new command. There are ba sically two different ways that the computer can elect as far as the time it takes to complete the mode switch. In one instance, the slew is accomplished in a fixed number of time periods and if the equalization is realized before the total number of counts, then there is literally a hunting action around the value of the equalized voltage that constitutes the new command. On the other hand, the transfer time need not be a fixed period, that is composed of a predetermined number of time intervals, to handle the equalizing, but can be only as long as required to force the DC error voltage representing the new command signal to equal the stored DC error signal. In this situation, the computer logic terminates the transfer time upon the first change of state of the slew input.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE exemplifying our invention depicts the prior art arrangement in block form with our improved control mode switching circuitry appearing within a dashed rectangle.

DESCRIPTION OF THE PREFERRED EMBODIMENT Although our invention is susceptible to use with respect to electromechanical servomechanisms, inasmuch as it finds particular utility in connection with hydraulic servo systems, such a system will be referred to. Consequently, a servomechanism denoted generally by the reference numeral 10 includes the servohydraulics and the transducer conditioners, which components are conventional as far as hydraulic servo systems are concerned, and are collectively designated by the block 12. It will be appreciated that the servo hydraulics represented by the block 12 include any preferred number of actuators (such as those manufactured by MTS Systems Corporation, the assignee of the invention herein described, under Model No. MTS Cylinder Series 204), there being one associated with each chan nel that is involved in a given situation, and the transducers may be either of the load cell type where force or load is to be measured or of the differential transformer type where position or displacement is concerned.

Actually, it is contemplated that the mode switching will involve a change, say, from a load control to a position or stroke control. Normally, owing to the difference in command values, this results in a perturbation on the new command parameter as the system is switched. It will be appreciated that the new command parameter might very well call for a reversal in the direction the servomechanism is moving.

More specifically, in a hydraulic servo system, the piston rod of the actuator might very well be moving in one direction, either at a constant velocity or where its acceleration deceleration is changing, yet the new command requires a reversal of the direction in which the piston rod is moving. Quite obviously, the specimen undergoing test receives a severe shock, the magnitude of which depends upon the difference in command values, when there is such an abrupt change and in the alluded to situation even a change in direction.

It will be appreciated that the servomechanism operates in a closed control loop manner. Accordingly, it further includes a command line 14 plus a feedback line 16. A summing junction 18 is employed as an error sensing device. Accordingly, one input terminal of the summing junction carries the reference numeral 18a, this being the one to which the command line 14 is connected, and a second input terminal 18b, this being the one to which the feedback line 16 is connected. The output terminal of the summing junction has been given the reference numeral 180. A gain controller assigned the reference numeral 20 is also employed. Actually, the summing junction 18 and gain control 20 constitute what is frequently referred to as the servo controller, this being purchasable as a unit.

Although the valve amplifier 24 can be part of the controller composed of the summing junction 18 and gain control 20, it also can be acquired separately and the separate denoting of the valve amplifier 24 facilitates the description of the conventional components comprising the control loop 10. It will be remembered that at the outset it was stated that a hydraulic servomechanism would be described. Thus, the DC error signal appearing at the output of the gain control 20 is simply amplified by the valve amplifier 24 and then applied to the actuator involved in the hydraulic servomechanism 10.

The servomechanism 10 in the illustrative instance additionally includes any number of transducer lines 26, the number depending upon the number of channels involved in the rectangle 10. Where a number of channels, and of course a number of transducer lines 26 are involved, then a multiplex switch 28 is employed for applying at the proper time the feed-back signal carried by a particular line 26 to the second input terminal 18b of the summing junction 18.

A digital computer 30 is shown in the drawing. A conventional digital-to-analog converter 32 connects the output ofthe computer 30 to the input terminal 18a of the summingjunction 18. More will be said concerning the role played by the D/A converter 32. At this time, though, attention is called to the presence of a logic line 34 extending from the computer 30 to the previously mentioned multiplex switch 28, the logic line 34 selecting the particular feedback signal that is to be applied to the input terminal 18b of the summing junction 18. Inasmuch as feedback information is to be fed into the computer 30, an analog-to-digital multiplexer 36 has its plurality of inputs connected to the various transducer lines 26 and its output to the input of an analog-to-digital converter 38, which is of conventional design. A logic line 40 extends between the computer and the multiplexer 36 for the purpose of selecting which feedback signal is to be read into the computer via the multiplexer and of course the A/D converter 38.

The foregoing description has dealt with a conventional system. The circuit generally designated by the reference numeral 50, and which is contained within the dashed rectangle, overcomes certain shortcomings that have already been mentioned. In this regard, the conventional arrangement involves the reading of the particular feedback voltage level on a particular transducer line 26 into the computer 30, this being done through the A/D converter 38. With the actual feedback voltage for a given transducer line 26 connected to the summing junction 18, this being accomplished through the multiplex switch 28, in a matter of microseconds thereafter the required command voltage level to match this feedback level is put out to the input terminal 118a of the summing junction 18, this being done via the D/A converter 32. However, any appreciable difference between the signals applied to the input terminals 18a and 1812 will be reflected in rather large perturbations as far as the servomechanism 10 is concerned. This, as previously pointed out, can cause a severe acceleration shock to the specimen undergoing test by the servomechanism 10. It is the basic purpose of the present invention to avoid the so-called bumps that occur by reason of a mismatch between the new command parameter or signal and the specific feedback signal. I

Consequently, it can be explained at this stage that the usual direct current error signal which is fed from the gain control 243 to the valve amplifier 24 is interrupted. The function of the circuit is to achieve this interruption, yet maintain a continued operation of the servomechanism 10. To achieve this function, the circuit 50 includes a track and store amplifier 52 comprising an operational amplifier 54, a resistor 55 and a capacitor 56.

By means of a field effect transistor 58, more specifically through the agency of a transfer line 59 extending to the gate thereof through a diode 60 from the output terminal 61c of a differential amplifier 611 having its first input terminal 61a connected to the'computer 30 via a transfer line 62. The other input terminal 60b is held at a fixed potential.

Consequently, when a digital signal representing a 0 is fed from the computer 30 over the transfer line 62, then the transistor 58 is rendered conductive so that the amplifier 52 tracks whatever error signal is being delivered from the gain control 20. On the other hand, when no signal is forwarded from the computer over the transfer line 62, then the transistor 58 is rendered nonconductive. It is at this time, though, that the particular error signal from the gain control 20 that has been delivered to the amplifier 52 is stored or held and the stored or held value at the output side of the amplifier 52 is applied to the valve amplifier 24 to cause the particular servomechanism to continue to operate at the same signal that it was operating at before the change of the transistor 58 to its nonconductive state.

Further included in the control circuit 50 is a comparator 63 having first and second input terminals 630 and 63b and an output terminal 63c. It will be observed that the input terminal 63a is connected by way of an input line 64 directly to the gain control 20 so that whatever signal is forwarded from the gain control 20 is impressed on the input terminal 63a. A second line 66 connects the input terminal 63b to the output side of the amplifier 52 Hence, the function of the comparator 63 is to compare whatever value of DC error there is from the gain control 20 with the stored DC error signal at the output of the amplifier 52.

A slew line 68 extends from the output terminal 630 of the comparator 63 to the computer 30. The comparator 63 will output a 0 signal whenever the value of the signal applied to the input side of the amplifier 52 is greater than the stored DC error signal and conversely a 1 signal when the stored error signal is greater than the signal delivered to the amplifier 52. The logical output, whether a 0 or 1 informs the computer 30 as to whether there should be an increase or decrease as far as the value of the command signal delivered to the D/A converter 32.

OPERATION Assuming that a switch is to be made from a load mode control to a stroke mode control, whatever the error signal at the output side of the amplifier 52 happens to be is stored. It is the value of this DC error signal that is to be matched by a new command signal or parameter to effect the smooth or bumpless change from the load mode to the stroke mode. It will be appreciated that the stroke mode is decidedly different from the load mode and that if a sudden switch between modes were attempted without any moderation, there could, and in all likelihood there would be, an abrupt change in the value of the command signal with respect to the feedback signal from the previous mode. This creates or causes the serious perturbation effect that has been already mentioned and which can adversely affect the specimen being tested.

Consequently, the track and store amplifier 52 maintains whatever DC error signal and this DC error signal continues to be the input to the valve amplifier 24 during the mode switching. In other words, where a hy draulic servomechanism is involved, as we have selected in this instance, the held or maintained error signal represents the velocity of the actuator and this same velocity will be maintained during the change from the first mode to the second mode. Irrespective of the value of the DC error voltage before the mode transfer is started, it is this value that the new command signal is to be adjusted to.

The computer software in the system controls the mode switch sequence. A decision to effect the switch can be, it will be understood, generated by the system operator through, say, via a teletype input or by the test software itself as a result of the test progress. Once the decision to switch has been made, the operating system subroutine within the computer then takes control and reads the new feedback voltage and calculates a new command level which is delivered from the computer 30 to the D/A converter 32.

The mode switch routine then outputs a digital word to cause the track and store amplifier 52 to go into its store phase of its operation. Thus, whatever error signal appears or has been appearing is held and as already explained this signal is utilized for controlling the servomechanism 10. It will be recognized, just to take an example, that the value of the feedback signal may have a magnitude or value of 5 volts. The particular transducer line 26 that is involved may be at only 1 volt. What has to take place is that the new command must change from 5 volts to 1 volt. When operating in real time, what happens is that the computer literally looks through the A/D converter 38 to the transducer voltage and under these conditions sees that it is 1 volt. It reads this into the computer and outputs it through the D/A converter 32. Owing to the fact that there are errors in the overall system, there is not an exact match of 1 volt unless everything is perfectly tuned, which nevers happens in actual practice. Therefore, the new command might be at 0.9 volt.

Inasmuch as there is a difference of 0.1 volt, the comparator 63 senses this difference and puts out a signal on the slew line 68 indicating that the new command must be increased. Assuming that the clock frequency is kilocycles, each step will be 5 microseconds. Each step will also involve a 5 millivolt shift. Hence, there will be a 100 microsecond period that it takes to increase the 0.9 volt to 1.0 volt to effect the desired equalization. After the equalization is reached, then the voltage is continually adjusted plus or minus by 5 millivolts until the entire time period has elapsed. Sixty-four steps has been found to be a very good time for the total period. However, this could be virtually anything, say, 32 if desired. It is best, though, to keep it on an octal number basis.

Consequently, the computer 30 has put out a reveised command on the basis of the slew requirements of the comparator 63. It has already been explained that a 0 from the comparator 63, that is from its output terminal 63c, would indicate a negative slew or an increase in the command in the negative direction. Inasmuch as a positive increase has been called for under our illustrative set of conditions, a 1 signal on the output terminal 63c causes an increase, which is proper under these circumstances.

After equalization of the two DC signals, the mode switch routine in the computer returns the track and store amplifier 52 to its track mode, a 0 signal being substituted on the transfer line 62 for the previously 1 signal that called for no transfer, which was what was required during the changeover from the first mode to the second mode.

At this stage, the computer software then takes control and applies whatever command signal is called for in order to accomplish the desired stroke control. Thus, a shift has been effected from the load mode to the stroke mode control and it is to be appreciated that this has been done without any sudden change. In other words, the transition has been smoothly realized and the so-called bumpless switch has been completed.

We claim:

1. A control mode switching system for a hydraulic servomehcanism controllable by load control mode or stroke control mode command signals and having means to provide load or stroke feedback signals, and

in which an error signal representing the difference between a command signal and a corresponding mode feedback signal is normally used to control the operation of the servomechanism, the system comprising means for changing from a present feedback signal to a new feedback signal of a different control mode, means responsive to actuation of said means for changing for maintaining the value of said error signal substantially constant for a time period subsequent to the operation of said means for changing, means for controlling the servomechanism in accordance with the value of the maintained error signal, said new feedback signal and a corresponding command signal providing a new error signal, means for comparing the maintained error signal and the new error signal, and means responsive to the means for comparing for changing the value of said new command signal during said time period until the new error signal changes in the direction of said maintained error signal to reduce the difference therebetween.

2. In combination with the control mode switching system set forth in claim 1, a computer, means for feeding information into said computer corresponding to the value of the new feedback signal, the computer subtracting the signal corresponding to the new feedback signal from the command signal to produce a new command signal with the consequence that said new error signal is provided in accordance with the difference between the value of the new command signal and the value of said new feedback signal, said means for comparing comprising a comparator for determining the difference between the new error signal and the maintained error signal, the comparator controlling the computer so as to change the new command signal in a direction to equalize the new error signal with the maintained error signal.

3. The combination of claim 2 in which said means for maintaining the value of said error signal substantially constant includes a track and store amplifier and a switch connected to said amplifier for causing said amplifier to track the error signal representing the difference between the value of said command signal and the value of said feedback signal, and means connected to the computer for causing said switch to be closed when said error signal is to be tracked and to be opened when said error signal is to be stored.

4. The combination of claim 3 in which said lastmentioned means includes a differential amplifier having a pair of input terminals, one of said input terminals being connected to the computer and the other of said input terminals being connected to a fixed potential, and said switch constituting a field effect transistor having its gate connected to said output terminal and its source and drain in circuit with said amplifier to forward said error signal to said amplifier when said transistor is conductive.

5. In combination with a servomechanism including a normally closed control loop having a summing junction provided with a first input terminal to which a command signal is applied and'a second input terminal to which a feedback signal from the servomechanism is applied, said summing junction having an output terminal for providing an error signal representative of any difference between the values of the command and feedback signals applied to said first and second terminals, a track and store amplifier having an input and an output, a switch means selectively connecting the output of said summing junction to the input of said track and store amplifier, a signal comparing means having a pair of inputs and an output, the output of said track and store amplifier being connected to normally control the value of the error signal, and also being connected to one input of the signal comparing means, a control mode switching system for said servomechanism comprising means changing to a different command signal because of change in control mode, means associated with the amplifier for storing a signal corresponding to the value of said error signal, said switch being responsive to the change of control mode to cause said track and store amplifier to deliver the stored signal for controlling the servomechanism in accordance with the stored signal for a period of time, and means connected to the output of said comparing means for adjusting the value of said different command signal during said period of time to provide an error signal having a value approximating that of the stored signal, said switch means returning said amplifier to track mode to track said error signal after said period of time.

6. The combination of claim 5 including a computer, said control mode switching system comprising a slew line connected between said comparing means and said computer for causing the computer to slew said different command signal so that said error signal at the output terminal of said summing junction is substantially equal to said stored signal.

7. The combination of Claim 6 in which said computer is a digital computer, and further including a digital-to-analog converter connected between said computer and the first input terminal of said summing junction.

8. The combination of Claim 7 further including an analog-to-digital converter for converting the value of the feedback signal to a digital signal for entry into the computer so that said computer can subtract the value of the feedback signal delivered to it from the then existing digital command signal to produce the new command signal.

9. In combination, a servomechanism including a valve amplifier for supplying fluid to said servomechanism and a feedback line carrying a signal representative of a measured condition of an actuator element contained in said servo-mechanism, a digital computer, an analog-to-digital converter connected between said feedback line and said computer, a summing junction having first and second input terminals and an output terminal, said feedback line being connected to said second input terminal, a digital-to-analog converter connected between said computer and said first input terminal to apply a command signal to said first input terminal, whereby an error signal representative of the difference between the value of the command signal and the value of the feedback signal appears at the output terminal of said summing junction, a track and store amplifier connected to said output terminal, means controlled by the computer for causing said track and store amplifier to either track the error signal appearing at said output terminal or to cause said amplifier to store the signal appearing at said output terminal, the output of said track and store amplifier being connected to said valve amplifier for delivering either the tracked or stored signal to said valve amplifier, a comparator having first and second input terminals and an output terminal, the first input terminal of said comparator being connected to the output terminal of said summing junction and the second input terminal of said comparator being connected to said track and store amplifier, a digital signal line connected between the output terminal of said comparator and said digital computer for controlling the computer to adjust the command signal delivered to said digital-to-analog converter so as to change the error signal at the output terminal of said summing junction so that it corresponds in value to that of the error signal stored by said track and store amplifier.

10. The combination set forth in claim 9 in which said means for controlling the track and store amplifier includes a differential amplifier having a first input terminal, a second input terminal and an output terminal, and a field effect transistor having its gate connected to the output terminal of said differential amplifier, the source of said field effect transistor being connected to the output terminal of said summing junction and the drain of said field effect transistor being connected to said track and store amplifier, the first input terminal of said differential amplifier being connected to said computer and the second input terminal of said differential amplifier being connected to a fixed potential.

11. In combination with a servomechanism, a digital computer having an input and an output, said computer supplying via its output various primary command signals representing the desired positions of a member actuated by said servomechanism, means providing a feedback signal representing the actual position of said member, summing means having a first input terminal connected to the output of said computer and a second input terminal connected to the feedback means and an output terminal for providing an error signal representing any difference between whatever command signal is applied to the first input terminal of said summing means and the feedback signal applied to the second input terminal thereof so as to provide an error signal at its said output terminal for controlling the position of said member, switch means for connecting said feedback means to the input of said computer to cause said computer to change the value of an existing primary command signal to a secondary command signal having a value approximating the difference between the value of said primary command signal and the value of said feedback signal, means for tracking and storing said error signal prior to the change from said primary command signal to said secondary command signal, means connected between said computer and said tracking and storing means for causing said tracking and storing means to store the error signal prior to said command signal change, comparator means having a first input terminal and a second input terminal and an output terminal, the first input terminal of said comparator means being connected to the output terminal of said summing means and the second input terminal of said comparator means being connected to said tracking and storing means, and means connected to the output terminal of said comparator means for causing the computer to increase or decrease the value of said secondary command signal so that it causes the resulting error signal appearing at the output terminal of said summing means to equal substantially the value of the stored error signal.

12. The combination set forth in claim 11 in which said computer supplies a second primary command signal indicative of a different desired position of said controlled member so that the value of the secondary command signal applied to the first input terminal of said summing means is changed incrementally to the value of the second primary command signal.

13. The combination set forth in claim 12 in which said computer is a digital computer and in which said feedback signal is an analog signal, the combination further including an analog-to-digital converter connected between said switch means and the input of said computer, and a digital-to-analog converter connected between the output of said computer and the first input terminal of said summing means, said summing means being an analog device. 

1. A control mode switching system for a hydraulic servomehcanism controllable by load control mode or stroke control mode command signals and having means to provide load or stroke feedback signals, and in which an error signal representing the difference between a command signal and a corresponding mode feedback signal is normally used to control the operation of the servomechanism, the system comprising means for changing from a present feedback signal to a new feedback signal of a different control mode, means responsive to actuation of said means for changing for maintaining the value of said error signal substantially constant for a time period subsequent to the operation of said means for changing, means for controlling the servomechanism in accordance with the value of the maintained error signal, said new feedback signal and a corresponding command signal providing a new error signal, means for comparing the maintained error signal and the new error signal, and means responsive to the means for comparing for changing the value of said new command signal during said time period until the new error signal changes in the direction of said maintained error signal to reduce the difference therebetween.
 2. In combination with the control mode switching system set forth in claim 1, a computer, means for feeding information into said computer corresponding to the value of the new feedback signal, the computer subtracting the signal corresponding to the new feedback signal from the command signal to produce a new command signal with the consequence that said new error signal is provided in accordance with the difference between the value of the new command signal and the value of said new feedback signal, said means for comparing comprising a comparator for determining the difference between the new error signal and the maintained error signal, the comparator controlling the computer so as to change the new command signal in a direction to equalize the new error signal with the maintained error signal.
 3. The combination of claim 2 in which said means for maintaining the value of said error signal substantially constant includes a track and store amplifier and a switch connected to said amplifier for causing said amplifier to track the error signal rEpresenting the difference between the value of said command signal and the value of said feedback signal, and means connected to the computer for causing said switch to be closed when said error signal is to be tracked and to be opened when said error signal is to be stored.
 4. The combination of claim 3 in which said last-mentioned means includes a differential amplifier having a pair of input terminals, one of said input terminals being connected to the computer and the other of said input terminals being connected to a fixed potential, and said switch constituting a field effect transistor having its gate connected to said output terminal and its source and drain in circuit with said amplifier to forward said error signal to said amplifier when said transistor is conductive.
 5. In combination with a servomechanism including a normally closed control loop having a summing junction provided with a first input terminal to which a command signal is applied and a second input terminal to which a feedback signal from the servomechanism is applied, said summing junction having an output terminal for providing an error signal representative of any difference between the values of the command and feedback signals applied to said first and second terminals, a track and store amplifier having an input and an output, a switch means selectively connecting the output of said summing junction to the input of said track and store amplifier, a signal comparing means having a pair of inputs and an output, the output of said track and store amplifier being connected to normally control the value of the error signal, and also being connected to one input of the signal comparing means, a control mode switching system for said servomechanism comprising means changing to a different command signal because of change in control mode, means associated with the amplifier for storing a signal corresponding to the value of said error signal, said switch being responsive to the change of control mode to cause said track and store amplifier to deliver the stored signal for controlling the servomechanism in accordance with the stored signal for a period of time, and means connected to the output of said comparing means for adjusting the value of said different command signal during said period of time to provide an error signal having a value approximating that of the stored signal, said switch means returning said amplifier to track mode to track said error signal after said period of time.
 6. The combination of claim 5 including a computer, said control mode switching system comprising a slew line connected between said comparing means and said computer for causing the computer to slew said different command signal so that said error signal at the output terminal of said summing junction is substantially equal to said stored signal.
 7. The combination of Claim 6 in which said computer is a digital computer, and further including a digital-to-analog converter connected between said computer and the first input terminal of said summing junction.
 8. The combination of Claim 7 further including an analog-to-digital converter for converting the value of the feedback signal to a digital signal for entry into the computer so that said computer can subtract the value of the feedback signal delivered to it from the then existing digital command signal to produce the new command signal.
 9. In combination, a servomechanism including a valve amplifier for supplying fluid to said servo-mechanism and a feedback line carrying a signal representative of a measured condition of an actuator element contained in said servo-mechanism, a digital computer, an analog-to-digital converter connected between said feedback line and said computer, a summing junction having first and second input terminals and an output terminal, said feedback line being connected to said second input terminal, a digital-to-analog converter connected between said computer and said first input terminal to apply a command signal to said fiRst input terminal, whereby an error signal representative of the difference between the value of the command signal and the value of the feedback signal appears at the output terminal of said summing junction, a track and store amplifier connected to said output terminal, means controlled by the computer for causing said track and store amplifier to either track the error signal appearing at said output terminal or to cause said amplifier to store the signal appearing at said output terminal, the output of said track and store amplifier being connected to said valve amplifier for delivering either the tracked or stored signal to said valve amplifier, a comparator having first and second input terminals and an output terminal, the first input terminal of said comparator being connected to the output terminal of said summing junction and the second input terminal of said comparator being connected to said track and store amplifier, a digital signal line connected between the output terminal of said comparator and said digital computer for controlling the computer to adjust the command signal delivered to said digital-to-analog converter so as to change the error signal at the output terminal of said summing junction so that it corresponds in value to that of the error signal stored by said track and store amplifier.
 10. The combination set forth in claim 9 in which said means for controlling the track and store amplifier includes a differential amplifier having a first input terminal, a second input terminal and an output terminal, and a field effect transistor having its gate connected to the output terminal of said differential amplifier, the source of said field effect transistor being connected to the output terminal of said summing junction and the drain of said field effect transistor being connected to said track and store amplifier, the first input terminal of said differential amplifier being connected to said computer and the second input terminal of said differential amplifier being connected to a fixed potential.
 11. In combination with a servomechanism, a digital computer having an input and an output, said computer supplying via its output various primary command signals representing the desired positions of a member actuated by said servomechanism, means providing a feedback signal representing the actual position of said member, summing means having a first input terminal connected to the output of said computer and a second input terminal connected to the feedback means and an output terminal for providing an error signal representing any difference between whatever command signal is applied to the first input terminal of said summing means and the feedback signal applied to the second input terminal thereof so as to provide an error signal at its said output terminal for controlling the position of said member, switch means for connecting said feedback means to the input of said computer to cause said computer to change the value of an existing primary command signal to a secondary command signal having a value approximating the difference between the value of said primary command signal and the value of said feedback signal, means for tracking and storing said error signal prior to the change from said primary command signal to said secondary command signal, means connected between said computer and said tracking and storing means for causing said tracking and storing means to store the error signal prior to said command signal change, comparator means having a first input terminal and a second input terminal and an output terminal, the first input terminal of said comparator means being connected to the output terminal of said summing means and the second input terminal of said comparator means being connected to said tracking and storing means, and means connected to the output terminal of said comparator means for causing the computer to increase or decrease the value of said secondary command signal so that it causes the resulting error signal appEaring at the output terminal of said summing means to equal substantially the value of the stored error signal.
 12. The combination set forth in claim 11 in which said computer supplies a second primary command signal indicative of a different desired position of said controlled member so that the value of the secondary command signal applied to the first input terminal of said summing means is changed incrementally to the value of the second primary command signal.
 13. The combination set forth in claim 12 in which said computer is a digital computer and in which said feedback signal is an analog signal, the combination further including an analog-to-digital converter connected between said switch means and the input of said computer, and a digital-to-analog converter connected between the output of said computer and the first input terminal of said summing means, said summing means being an analog device. 